Grant: Subatomic Physics Envelope - Project

Nuclear beta decay correlation experiments helped establish the nature of the weak interaction, a theory with spin-1 bosons (like the photon, but massive) coupling only to left-handed leptons (i.e. electrons and neutrinos). The angular distribution of the outgoing particles, if measured to part per thousand, is sensitive to new interactions that can produce leptons with different handedness. We have measured the beta asymmetry with respect to the nuclear spin in 37K decay to be Abeta=-0.5706+-0.0018. This was C.S. Wu's original parity-violating observable, and our beta asymmetry measurement has the best fractional accuracy (i.e. smallest uncertainty/Abeta) in any nucleus or the neutron, and is in agreement with theory. Our average value of Abeta can either constrain interactions producing wrong-handed neutrinos, or, combined with other experiments, tests whether nuclei with finite spin and magnetic moment all have the same intrinsic strength of beta decay. We will further explore this possible "conserved vector current" violation by measuring Abeta and other observables better in 37K. The dependence of Abeta on beta energy is still under analysis by a Ph.D. student, and is separately sensitive to new 4-fermion effective field theory interactions. Our proposed measurements, including the asymmetry of the direction of progeny nuclei with respect to the intial nuclear spin, probe new interactions producing wrong-handed positrons: these observables scale with a helicity factor (electron mass)/(electron energy) that is typically suppressed at higher energy, making our measurements complementary. Key to our measurements is our demonstrated ability to highly polarize the nuclei with our atom trap techniques, and to determine the polarization of the nuclei decaying to 0.1% accuracy by atomic methods in situ. We also plan a time reversal symmetry test using the three independent momenta of the products of radiative beta decay, a unique test in the first generation of particles at relatively low energy scale. Reactor neutrinos show anomalies in their total number and energy dependence. We are reconstructing the neutrino energy spectrum from 92Rb 0- to 0+ decay from the momenta of the other products, using only the beta direction and not its energy. Several 0- to 0+ decays together produce about 30% of the higher-energy neutrinos from a reactor, and while in principle the theory is reasonably defined in terms of two nuclear structure-dependent matrix elements, there is inconsistency in the literature between difficult beta energy spectrum measurements in this type of decay. Simultaneous measurement of the beta-neutrino correlation will help us extract the two matrix elements to better test the 1st-forbidden theory. We expect to continue to maintain TRIUMF EEC high priority experiments for 2 graduate students at a time, so ask for additional funds to start a 3rd student while the 2nd student completes the analysis part of a Ph.D.